Date: Sat, 10 Oct 2015 15:53:48 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Ed Maste <emaste@freebsd.org> Cc: Dimitry Andric <dim@freebsd.org>, "src-committers@freebsd.org" <src-committers@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org> Subject: Re: svn commit: r289072 - in head/contrib: libc++/src llvm/lib/Transforms/Vectorize Message-ID: <20151010125348.GF2257@kib.kiev.ua> In-Reply-To: <CAPyFy2BsS3Z7izEtWGqyXJkBcoykOX5y144w4Q997TKwWxaYDQ@mail.gmail.com> References: <201510091821.t99ILjHI054211@repo.freebsd.org> <20151010060846.GA2257@kib.kiev.ua> <CAPyFy2Dm=OaJbTfwVDABPz=e_H-XhhnTc%2BjYtr0iSHYv3uEObw@mail.gmail.com> <20151010120311.GE2257@kib.kiev.ua> <CAPyFy2BsS3Z7izEtWGqyXJkBcoykOX5y144w4Q997TKwWxaYDQ@mail.gmail.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On Sat, Oct 10, 2015 at 12:24:33PM +0000, Ed Maste wrote: > On 10 October 2015 at 12:03, Konstantin Belousov <kostikbel@gmail.com> wrote: > >> The declaration is extern ostream cout; which correctly has 8 byte alignment. > > The alignment of the declaration does not matter, most likely. > > I am not sure what you mean by saying that 8 byte alignment is desirable, > > while compiler generates accesses (movdqa) which assume 16-byte alignment. > > I mean that this object is naturally expected to have 8-byte > alignment. In previous versions of libc++.so shipped with FreeBSD it > did, and can't be changed now (cout comes from the binary itself, via > a copy relocation). No, the natural alignment for char[] is 1, while ABI requires an array longer than 15 bytes to have 16-bytes alignment. > > >> In the implementation (contrib/libc++/src/iostream.cpp) it is defined > >> as a char array which is where the 16-byte ABI alignment would arise, > >> but explicitly specifies 8-byte alignment: > >> _ALIGNAS_TYPE (ostream) _LIBCPP_FUNC_VIS char cout[sizeof(ostream)]; > >> > >> Compiling with -emit-llvm --save-temps shows the desired 8-byte > >> alignment for cout in the LLVM IR but 16-byte in the assembly file. > > Could somebody show the readelf -s iostream.So and readelf -s libc++.so.X > > output lines for the symbol, please ? > > readelf -s iostream.So: > 120: 0000000000000150 160 OBJECT GLOBAL DEFAULT 5 _ZNSt3__14coutE > > libc++.so.1.full > 1280: 00000000002bb4a0 160 OBJECT GLOBAL DEFAULT 25 _ZNSt3__14coutE And there the object has right alignment.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20151010125348.GF2257>